The present invention relates to thermal ink jet printing and more particularly to the manufacture of a plastic orifice plate for an inkjet printhead assembly, manufacture of an inkjet printhead assembly, provision of a plastic orifice plate and provision of an inkjet printhead assembly.
In thermal inkjet printing, localised heat transfer to a defined volume of ink, which is located adjacent to an ink jet orifice, vaporises the ink and causes it to expand thereby ejecting the ink from the orifice during the printing of characters on a print medium. The defined volume of ink is usually provided in a xe2x80x9cbarrier layerxe2x80x9d which provides a plurality of ink reservoirs. These reservoirs are located between a corresponding plurality of resistive heater elements, usually provided by a thin film structure, and a corresponding plurality of orifices (which are effectively nozzles), provided by an xe2x80x9corifice platexe2x80x9d.
Thus orifice plates with multiple orifices aligned with thin film resistors are used to control the trajectory, drop weight and drop velocity of ink drops. Typically, these orifice plates are manufactured by electroforming processes and the metal that is commonly used is Nickel. Details of such metallic orifice plates and the functioning and manufacture of thermal inkjet printheads with orifice plates are described in the Hewlett-Packard Journal, Vol. 36, No.5, May 1985 and in U.S. Pat. No. 4,694,308 issued to C. S. Chan et al.
Use of plastic materials to fabricate orifice plates has certain advantages over metallic orifice plates. Some of the advantages of these plastic orifice plates are described in U.S. Pat. No. 4,829,319 issued to C. S. Chan et al. These include low cost of the orifice plates, transparency of the orifice plate which helps in viewing the fluid dynamics in the print cartridges, corrosion resistance to ink chemicals and the possibility of forming integral barrier layers on the thin film resistors.
U.S. Pat. No. 4,829,319 to Chan et al (hereafter US ""319) discloses a plastic orifice plate for an inkjet printhead and manufacturing process therefor which includes electroforming a metal die having raised sections thereon of predetermined centre-to-centre spacings, and using the die to punch out openings in a plastic substrate of a chosen thickness to form a plurality of closely spaced orifice openings in the substrate. However the process of US ""319 has a number of problems associated with it. First, it is difficult to preserve the structural integrity of thin plastic sheets during the die stamping operation. The thin plastic sheets are difficult to handle and are susceptible to tearing. Second, for most inkjet printing applications, a dimensional accuracy within sub-micron range is needed for the orifices and the US ""319 process may not give this level of accuracy. Third, the shape of the orifices is important in controlling the directionality of ink droplets and it is difficult to achieve a perfect shape definition with the US ""319 die stamping process. Fourth, the latest printheads require a high density of orifices in an orifice plate. This requires spacing consecutive orifices a distance of less than 10 microns apart, which spacing cannot be easily achieved using the US ""319 process. Fifth, the US ""319 process is rather complex involving many process steps, which may result in low yields in the process.
An object of the present invention is to provide a process for manufacturing plastic orifice plates which reduces at least some of the above problems. The invention includes providing a plastic orifice plate as such and also providing an inkjet printhead assembly which incorporates a plastic orifice plate.
The invention involves the use of a photoimageable polymer and photolithography for forming a plastic orifice plate having a defined pattern of orifices therein.
In another aspect, in forming an inkjet printhead assembly, a thin film resistor structure having a plastic barrier layer is provided and a formed plastic orifice plate is bonded thereto using heat and pressure.
Use of a photolithographic technique according to the invention allows use of a substrate to support a photoimageable polymer layer for the photolithographic steps, thereby avoiding the problem of damaging the plastic sheets as in US ""319. Photolithography also allows for greater accuracy in the final product, both dimensionally and in orifice shapes, than is achievable in the US ""319 process. The invention also involves less process steps compared to the US ""319 process and thus should result in higher process yields.
For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.